1. Field of the Invention
The present invention relates to a resin composition for production of an optical element and, more particularly, to a resin composition for production of an optical element which, by defining the resin composition with the parameters associated with the viscoelasticity, when an optical element formed with this resin composition is used, prevents the surface thereof from being scraped away, being worn, or being crushed by application thereto of a pressure, as much as possible.
Also, the present invention relates to an optical element which, in a case where using the optical element in combination with other optical members such as a lenticular lens sheet, can prevent the surface of such other optical member from being scraped off or worn as much as possible and to a projection screen which comprising a combination of the optical element and the lenticular lens sheet.
2. Description of the Related Art
An optical element has an optical element surface produced by a layer of resin composition having imparted there to an optical configuration being laminated on a transparent base member, or by an optical configuration being imparted directly onto such layer, only, of resin composition. While as the optical configuration of the optical element surface there are a wide variety of optical configurations, in the optical element formed using resin composition, in many cases, it has a surface configuration wherein fine lens configurations are arrayed and which, therefore, when viewed as a whole, comprising a large number of concavities and convexities.
Incidentally, when using the optical element, it is sometimes used by combining a plurality of optical elements. When using the optical element by combining two or more optical elements, in order to exhibit their effects to the maximum extent and also concurrently protect the optical element surfaces, locating the optical elements closely to each other with their surfaces opposing each other is often performed.
The most typical example is the case of a Fresnel lens sheet and lenticular lens sheet in a projection screen, in which, ordinarily, the both sheets are used with the Fresnel lens (circular Fresnel convex lens) surface and the lenticular lens surface being located closely to each other.
When locating the optical elements with their respective opposing surfaces being disposed closely to each other in the above-described way, since the both surfaces are a concavo-convex surface, they affect their opponent""s surfaces.
For example, in the above-described typical example, the cross-sectional configuration of the Fresnel lens surface is in the form of a saw-tooth like configuration and sharpens at its forward ends. On the other hand, the lenticular lens has its cross section rounded into a circular or elliptical, i.e. embossed, configuration. When such Fresnel lens sheet and lenticular lens sheet have been located closely to each other, the embossed apex of the lenticular lens and the sharpened forward end of the Fresnel lens are point-contacted with each other. Therefore, it can happen that, due to the contact pressure, the deformation of the lenticular lens and/or Fresnel lens, i.e. the crushing of the configuration thereof, will occur.
The above-described crushing of the lens configuration can indeed be prevented by increasing the hardness of the resin forming the lens. However, simply increasing that hardness results in that, when handling, or sheet-cutting, the lens becomes likely to chip off, and thus conversely causes a problem. Therefore, it is preferred to leave the viscosity not fully removed while the hardness is increased.
In addition, although the hardness of resin in general is closely related to the glass transition temperature (Tg), excessively decreasing the glass transition temperature (Tg) of resin becomes unable to obtain the rubber elasticity, with the result that applying a pressure causes the resin to get plastically deformed. Ordinarily, if the resin has some extent of crosslink density, even when the glass transition temperature is low, the rubber elasticity takes effect and, therefore, even when a pressure is applied, the resin does not get plastically deformed. In the case of the resin composition for production of an optical element, attempting to decrease the glass transition temperature and simultaneously to increase the crosslink density results in that introducing a high integrity of chains, which comprising a benzene ring or the like, is needed for the purpose of enhancing the refractive index which is requisite as the resin composition for production of an optical element. This becomes a cause of elevating the glass transition temperature. Conversely, regarding the method of increasing the glass transition temperature, although it is advantageous in terms of the enhancement of the refractive index, excessively increasing the glass transition temperature makes the rigidity excessively high, which may also cause a curving of the lens sheet.
Also, the optical element is not always used at normal temperature. When used in optical appliances or display devices, the production of heat from within the appliance or device can leave the optical element exposed to a high temperature. Also, when the product is shipped from the production factory and is in transit, it is possible that the temperature within the container or the hold will become 60xc2x0 C. or so. At these high temperatures, the above-described drawbacks will become more serious.
It is therefore an object of the present invention to provide a resin composition for production of an optical element which can solve over the above-described wide temperature range the drawbacks that include the respect that, conventionally, when the fellow optical element surfaces of the optical elements are located closely to each other, there is the possibility that the configuration of the optical element would be crushed and deformed due to the contact pressure; the respect that even if simply increasing the hardness, the optical element surface becomes likely to chip off and there is therefore the possibility that when handling or sheet-cutting a problem would be caused; the respect that when excessively decreasing the glass transition temperature of the resin, it becomes impossible to obtain the rubber elasticity and resultantly when a pressure is applied the resin gets plastically deformed; the respect that even when attempting to solve this drawback by increasing the crosslink density, because it is needed to enhance the refractive index, decreasing the glass transition temperature is hindered; and the respect that when conversely making it excessively high, because the rigidity becomes excessively increased, it results that a curving of the lens sheet occurs.
It is another object of the present invention to provide an optical element which has been formed with the thus-improved resin composition for production of an optical element.
The above-described objects have been attained by determining the relationship between the loss factor (tan xcex4; loss tangent) of the resin composition and the temperature and defining the range of the peak width, or, further, defining the range of the temperature (the temperature capable of reflecting the glass transition temperature) corresponding to the peak of the loss factor.
According to a first aspect of the present invention, there is provided a resin composition for production of an optical element being adapted to form the optical element, wherein, when W1/2 (xc2x0 C.) represents, regarding a crest portion in a loss factor/temperature curve obtained by measuring the loss factor of the resin composition relative to a change in the temperature, the width of the crest portion at the position of xc2xd of the maximum value of the loss factor in the crest portion temperature range; W0.1 (xc2x0 C.) represents, regarding the crest portion, the width of the crest portion at the position of 0.1 of the maximum value of the loss factor; and xcex94W (xc2x0 C.), the difference between the widths of the two crest portions, is set to be xcex94W=W0.1xe2x88x92W1/2, the xcex94W is in the range of from 16xc2x0 C. to 31xc2x0 C.
According to a second aspect of the present invention, there is provided a resin composition for production of an optical element being adapted to form the optical element, wherein, when W1/2 (xc2x0 C.) represents, regarding a crest portion in a loss factor/temperature curve obtained by measuring the loss factor of the resin composition relative to a change in the temperature, the width of the crest portion at the position of xc2xd of the maximum value of the loss factor in the crest portion temperature range; and W0.1 (xc2x0 C.) represents, regarding the crest portion, the width of the crest portion at the position of 0.1 of the maximum value of the loss factor, W0.1/W1/2 representing the ratio between the widths of the two crest portions is in the range of from 1.4 to 2.7.
According to a third aspect of the present invention, there is provided a resin composition for production of an optical element being adapted to form the optical element, wherein W1/2 (xc2x0 C.) representing, regarding a crest portion in a loss factor/temperature curve obtained by measuring the loss factor of the resin composition relative to a change in the temperature, the width of the crest portion at the position of xc2xd of the maximum value of the loss factor in the crest portion temperature range is in the range of from 13xc2x0 C. to 50xc2x0 C. and the value TP(xc2x0 C.) of the temperature at the maximum value of the loss factor is in the range of from 18xc2x0 C. to 47xc2x0 C. or from 52xc2x0 C. to 80xc2x0 C.
A fourth aspect of the present invention provides a resin composition for production of an optical element according to anyone of the first to third aspects, wherein the maximum value of the loss factor is in the range of from 0.4 to 1.2.
According to a fifth aspect of the present invention, there is provided an optical element the whole or a part of which is formed with the resin composition for production of an optical element as described in any one of the first to fourth aspects.
A sixth aspect of the present invention provides an optical element according to the fifth aspect, wherein the optical element is a Fresnel lens sheet.
According to a seventh aspect of the present invention, there is provided a projection screen which comprising the optical element as described in the fifth or sixth aspect and a lenticular lens sheet.